The present invention relates to a piston with a valve arrangement for a vehicle hydraulic brake system comprising a piston channel penetrating the piston, a valve seat formed around an opening of the piston channel, and a valve element, which is displaceable relative to the piston and for sealing the piston channel is positionable in a fluid-tight manner against the valve seat, wherein the sealing seat takes the form of an annular projection on a sealing component, which comprises an elastic sealing element and a support element, which stabilizes the sealing element.
Such a piston is known from EP 0 607 370 B2, and corresponding U.S. Pat. No. 5,473,896 which is incorporated by reference herein, and according to this background art is installed in a master brake cylinder of a vehicle brake system. The piston together with the master brake cylinder encloses a pressure chamber. In a ready position prior to a brake actuation, the valve arrangement is in an open position, in which the pressure chamber communicates with a hydraulic fluid reservoir. When a driver of the vehicle initiates a braking operation, the piston is displaced inside the master brake cylinder. In said case, the valve arrangement closes in that the valve element positions itself in a fluid-tight manner against the sealing seat, thereby interrupting the fluidic connection between the pressure chamber and the hydraulic fluid reservoir. Consequently, a high hydraulic pressure builds up in the pressure chamber and leads to actuation of the vehicle brake circuit and to activation of the vehicle brakes. On completion of the braking operation, the piston is moved back into its ready position, wherein the valve arrangement opens and the pressure in the pressure chamber reduces.
In modern vehicle brake systems, in addition to the previously described braking-induced pressure increase, considerable pressure increases moreover arise in the pressure chamber when automatic brake pressure generating systems, such as e.g. a vehicle traction control system or a vehicle stability control system, are activated. Such automatic brake pressure generating systems are used for selective actuation of a brake circuit independently of an active braking operation by the driver in order to activate individual wheel brakes for increased vehicle safety. The pressure increase in the pressure chamber is effected, for example, by means of an additional hydraulic pump. The pressure thus increased then has to be reduced by opening the valve arrangement.
It has been shown that the elastic sealing element according to the background art in the region, in which it is exposed to the pressurized hydraulic fluid in the pressure chamber, has a tendency to deform elastically and, especially given high hydraulic pressures, to “flow”. If, given high hydraulic pressure in the pressure chamber, the piston is moved back into its ready position, then, as the valve arrangement starts to open, i.e. as the valve element starts to move, the sealing element deforms in the region of the sealing seat under the action of the hydraulic pressure in such a way that the sealing seat expands and moves, for part of the lift of the valve element, together with this valve element. Allowance has to be made for this behaviour of the sealing element when designing the valve arrangement. It is therefore necessary to provide a large enough lift to guarantee reliable opening of the valve arrangement despite the pressure-related deformation of the valve seat. The valve element however has to complete this lift also during initiation of a braking operation, thereby delaying the response of the brake system.
In order to prevent such pressure-induced deformations at the sealing element, it is further known from EP 0 607 370 B2 to provide an additional valve, which during a pressure build-up separates the pressure chamber from the valve arrangement and therefore prevents high hydraulic pressures at the valve arrangement. This solution is however considerably more costly to manufacture and more susceptible to faults when in operation.
DE 39 32 248 A1 and U.S. Pat. No. 2,136,835 each disclose a piston with valve arrangement, in which piston the valve element during a pressure build-up is pressed by an annular bead into the sealing element. These solutions are susceptible to wear owing to the high mechanical loads acting upon the sealing element.